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CURRENT TRANSFORMER
Definitions
A device used to step down current from high value (primary current)to a low value (secondary current).
Low value current is easy to measure, no need to develop highcurrent measuring equipment.
The current ratio (primary current/secondary current) is substantiallyconstant for a given range of primary current.
The phase angle error (secondary current compared to primarycurrent) is within specified limit.
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Function
1. ProtectionCurrent transformer connected to protection equipment(protection relay). The protection relay can be set to identifywhether the input current (current transformers secondarycurrent) indicate that fault occur in the system.
2. MeasurementCurrent transformer connected to measurement equipment(ammeter, energy meter). Measure the current flow in the circuit.
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Construction Type
1. Ring or WindowThe popular type is ring shape, rectangular shape and ovalshape.
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Construction Type
2. Split CoreThe shape similar to ring or window construction, the core aresplit into two part.
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Construction Type
3. Primary Wound (Block)For low current rating. both primary and secondary are Woundover the core.
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Schematic Diagram
1. Single Ratio
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Schematic Diagram
2. Double RatioTransformer with an intermediate tapping on secondary winding.(terminal S1, S2 & S3) Ratio of tapping as per designrequirement. During service, unused tapping remain open.
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Schematic Diagram
3. Multi Ratio
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Location In The Single Line Diagram
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Connection to Measurement or Protection Devices
Measurement or protective devices are load to the currenttransformer. The circuit connection shall be in series.
Current transformer not connected to the load when installed to theswitchgear shall be shorted to ground (secondary terminal connectedto ground).
Open circuit is a condition when the current transformer is not
connected to its load. Open circuit during availability of primarycurrent will result of high voltage appear at the secondary terminal(>1kV A.C).
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Connection to Measurement or Protection Devices
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Typical Current Transformer Description
3000/1A - Current RatioCL.: 0.2 - Accuracy Class15VA - Rated Burden
3000/1A - Current RatioCL.: 5P20 - Accuracy Class & Accuracy Limit Factor30VA - Rated Burden
3000/1A - Current RatioCL.: X - Accuracy ClassVkp: 300V - Knee Point VoltageRct: 12 - Burden (Resistance)
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Term & Definitions
Rated Primary CurrentThe value of primary current which specified by manufacturer (shallbe based on customer/purchaser requirements). E.g. 3000A, 1600A,450A, etc.
Secondary CurrentThe value of secondary current which specified by manufacturer (hasbeen standardized). E.g. 1A or 5a.
Current RatioThe ratio of Rated Primary Current to Rated Secondary Current. E.g.3000/1A, 1600/5A. etc.
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Term & Definitions
Rated BurdenThe burden assigned by manufacturer at which the currenttransformer performs with specified accuracy expressed in VA.Connected burden shall be less than specified rated burden.
BurdenTotal load connected across the secondary of current transformerexpressed in VA or at rated secondary current.
Accuracy ClassThe class assigned to the current transformer with the specifiedlimits of ratio error and phase angle error. Accuracy class specifiesby BS & IEC standard. E.g. X, 5P, 10P, 0.2, 0.5, 1,etc.Class X, 5P, 10P for protection. Class 0.2, 0.5 for measurement.
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Term & Definitions
Accuracy Class
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Term & Definitions
Accuracy Limit FactorE.g. 10, 20.
Knee Point VoltageThe point at which C.T is saturate where 10% increase in voltagecauses a 50% increase in magnetizing current.
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Testing
Polarity TestTo prove that a group of C.Ts has been manufactured and installedwith the correct polarity.
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Testing
Magnetization CurveTo determine the secondary voltage and current at which the C.Tmagnetic core begin to saturate. The Result should be compared tomanufacturers curve.
The point at which saturation is deemed to become significant isknown as Knee Point Voltage, where 10% increase in voltagecauses a 50% increase in magnetizing current.
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Testing
Magnetization Curve
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Testing
Magnetization Curve
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Testing
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Testing
Burden CheckMeasurement of secondary resistance at terminal SI-S2 with anohmmeter. The value shall be as manufacturer specification.
Insulation Resistance TestCarried out when the wiring between C.T and the connectedequipment completed. To measure the C.T insulation resistance. Thevalue shall be as manufacturer specification.
Primary InjectionShould be carried out when the wiring is completed. To enablemeasurements to be made of the secondary quantities as verificationof the correct installation of the C.T, the connected wiring andequipment.
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Typical Installation Inside The Gas Insulated Switchgear
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Typical Installation Inside The Gas Insulated Switchgear
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Typical C.T Construction
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Typical complete assembly
C.T without outer protection
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Typical C.T Construction
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Typical C.T Construction
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Typical C.T winding machine
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